The most frequent use of optical fibers is to transmit modulated communication signals from a signal generator to a signal receiver. Optical fibers for transmitting such signals are generally quite thin, typically with fiber cores ranging from about 50-300 micrometers in diameter. A substantial amount of art exists relating to fiber optic cables which protect such optical fibers from various stresses to which they might be subjected, for example, my U.S. Pat. Nos. 4,143,942 and 4,147,406.
A somewhat less frequent use of fiber optic cables is for direct transmission of a light signal to be directly observed. In monitoring certain operations, the accepted safe test methods may require direct reading of visible light signals. In one very important application, monitoring coal-fired boilers, the only accepted method of monitoring water level is by light signal devices in which a beam of light is directed through a region which will contain water or steam depending on the water level within the boiler and through a prism which causes a color change at the beam target depending on the presence or absence of steam in the light transmitting region. For example, the light signal device may register a green color when water is present at the region and a red color when steam is present.
In larger boiler operations, the control room from which the coal-fired steam boilers are operated, may be substantially remote from the boiler, e.g., several floors apart. The currently accepted methods of transmitting the signal from the light signal device within the boiler are mirror arrangements, which in certain installations may be quite cumbersome, and closed circuit television camera monitors.
Fiber optic cable as a means of directly transmitting a visible light signal is more reliable and more convenient than currently used signal transmission methods. However, certain factors must be taken into account in the design of a fiber optic cable for transmitting visible light signals from a steam boiler that are different than the factors generally present in transmitting modulated optical communication signals. The fibers for transmitting a visible signal are typically much larger, e.g., 400-800 micrometers in diameter, and inherently much less flexible than optical fibers used for transmitting modulated communication signals.
A cable for transmitting a signal from a steam boiler or other extreme temperature environment is often subjected to widely different temperature environments throughout its length; for example, a cable for reading the water level within a steam boiler may be subjected to a temperature of 105.degree. C. within the boiler area and extend to a control room passing through an outside area where the temperature may drop to -40.degree. C. The wide temperature range to which the cable is subjected results in differential expansion of the materials which comprise the cable, and materials should be selected for use in such cable with regard to the temperature-related changes these materials have on the light transmission attentuation of the optical fibers.
Optical fibers consist of a central glass core, through which the light rays are actually transmitted, and means to retain the light within the central core, such as a surrounding cladding having a lower refractive index than the core so that a core-cladding interface tends to reflect rays back into the core rather than penetrate the barrier to become lost from the optical fiber. The transmittance of the optical fiber depends to a large extent on the uniformity of the core-cladding interface. Light transmitting through an optical fiber travels in different modes, that is, at differing angles with respect to the axis of the core. Lower order light modes pass through the fiber at minimum angles with respect to the core axis, striking the core-cladding interface at low incident angles and reflecting back into the core. Higher order light modes pass through the fiber at greater angles with respect to the axis of the core, and hence strike the interface at greater incident angles and also travel a greater total distance through the fiber. These factors contribute to higher order light modes being relatively quickly lost from the fiber while lower order modes may pass through a substantial fiber length without significant attenuation. The light transmission attenuation of an optical fiber is a function of the uniformity of the core-cladding interface because distortions in this interface regenerate more easily attenuated higher order light modes from lower order light modes.
Light attenuating distortions in the core-cladding interface may arise if the cable's optical fibers are subjected to differential stress throughout their length. Differential stresses on the fibers may arise when the cable is subjected to wide temperature variations throughout its length as a result of differential thermal expansion and contraction of the various materials of which the core is formed according to their various coefficients of thermal expansion. The differential stresses may either be radial, as a result of surrounding cable material pressing inwardly differentially on the optical fibers, or longitudinally, as a result of surrounding material expanding or contracting differentially relative to the optical fibers. For a cable which is to be subjected to wide temperature swings throughout its length, it is necessary to isolate the optical fibers from the effects of differential expansion and contraction of the materials as much as possible to minimize attenuation of light transmitted through the fibers.